The invention herein relates to the coating of substrates such as flexible plastic film which primarily is arranged in lengths by moving the substrate through a vessel in which some type of coating is applied, primarily by plasma vapor techniques.
Many structures in use today comprise thin members which have been uniformly coated with one or more substances whose characteristics and properties differ substantially from those of the members. The member receiving the coating is usually called a substrate and the coating is applied by one of several different techniques. Such structures include photographic film and paper, optical articles, other photosensitive structures, decorative objects and the like. The techniques by which the coatings are applied include vacuum deposition and plasma vapor techniques, these latter two techniques taking place inside of hermetic vessels whose atmospheres have been altered, by pumping down to achieve vacuum, by introducing atmospheres of different types of gases, etc.
For the purposes of the discussion and description which follow, several definitions should be kept in mind. The expression "thin film" as used herein is intended to mean a layer of some substance such as a semiconductor or ohmic material applied to a surface. Such a thin film layer is one which has a thickness that is measured in several thousands of Angstroms, such as for example 5000 A or 0.5 micron. The techniques and apparatus to be discussed make it feasible to deposit thin film layers whose thicknesses can be measured in even smaller fractions of microns.
Another expression which will be used is "photographic film." This means a complete article which includes a base of some plastic material such as sheeting, carrying an emulsion coating or the like. The article which is produced by the methods and apparatus of the invention can be considered a photographic or electrophotographic film which comprises a plastic sheet having one or more thin films deposited thereon.
The word "film" with its generic meaning as a thin coating or thin article without a modifying adjective will not be used herein in order to avoid confusion. While the substrate which is coated with thin film according to the method and apparatus of the invention may be called a "film" of plastic, since it is preferably of the order of 0.125 millimeters, it will be referred to herein only as a substrate. Reference has been made previously to a "flexible plastic film" for the purpose of giving an example of the type of substrate which is intended for use with the invention. No limitations are intended by the expression which has only been used for introduction purposes.
The invention herein is primarily concerned with the coating of thin flexible substrates arranged in strips or elongate members in an atmosphere of some inert gas such as argon using a thermal phenomenon which is known as plasma vapor deposition. While the technique may use nonelectrical apparatus, it is preferred that the plasma be created electrically. The method is commonly known as sputtering.
Reference made herein to "plasma" will mean an ionized gas created in a d.c. or radio-frequency a.c. field for the purpose of sputtering atoms from a target onto a substrate. The word "vapor" as used herein will mean a cloud of atomic particles created in a low pressure atmosphere by means of thermal evaporation or an electron gun, the vapor condensing on the surface of the substrate. Accordingly, "plasma vapor" means the cloud of particles created by sputtering or evaporation.
In the sputtering technique, a hermetic vessel is evacuated and then filled with an inert noble gas such as argon. The vessel is equipped with a cathode or target made of the material to be deposited and a substrate holder between which a high voltage a.c. or d.c. electrical field is established. The vessel may be made of stainless steel or heat resistant glass so as not to react with the sputtered material or the substrate. The target usually consists of a disc carefully brazed or otherwise bonded to a target holder which is cooled by a suitable coolant such as water to maintain a constant temperature. Various means are used to control temperature. Further features of the usual apparatus include high voltage conduits to bring either a.c. or d.c. power to the target from sources on the exterior of the vessel. The voltages fall typically in the kilovolt range. The substrate holder is in juxtaposition to the target and is also cooled to maintain its own temperature which may differ from the cathode or target temperature. This holder will usually have its own electrical connections but also will normally provide for the adjustment of the space between the target and the substrate to compensate for the gradual thinning of the target or to vary the deposition rate as a function of distance.
Since the electrical field may be either d.c. or a.c., the coating of either single or composite materials is capable of being achieved. D.c. sputtering is normally used for single conductive material targets whereas the deposition of composite materials which may consist of three or four elements necessitates a.c. sputtering at radio frequencies.
If the vapor pressures of the individual elements in a composite target are substantially different, then the element with the highest vapor pressure can be introduced in gaseous form. This technique will result in a reaction of the atoms flying from the target toward the substrate with the introduced gas during the flight, thus depositing a stoichiometrically accurate compound as the surface coating. This method is called reactive sputtering.
The presence of the high voltage electric field ionizes the inert gas thereby producing ions of a type which knock atoms from the target in addition to heat-producing secondary ions which have no practical value. The atoms knocked from the target are driven to the substrate and deposited. The process involves multiple collisions and complex physical effects but the net result is the depositing of the metal or other substance which comprises the target upon the substrate. Cooling means are used to offset the effect of the secondary electrons.
In vacuum depositing, a cloud of atoms is created thermally or by means of an electron gun within the pressure vessel under vacuum and the cloud condenses upon anything in the vessel including the exposed surface of the substrate. Cooling means normally are used to keep the substrate temperature within limits which retain its stability.
The invention herein will be discussed in connection with a method of and apparatus for sputtering, but no limitations are intended thereby. Important aspects of the invention are applicable to techniques for thin film deposition by methods in addition to sputtering.
Devices known in the prior art for the coating of substrates according to the general technique referred to are of two general configurations:
1. A vessel which may contain one or several substrate samples arranged in such a fashion as to facilitate batch coating; and PA1 2. A vessel characterized by an entrance and an exit port through which a length of substrate moves in a continuous fashion.
The first of these structures and techniques uses a vessel which represents a closed chamber in which the conditions for plasma vapor deposition have to be reestablished each time that a new batch of substrates is loaded into the vessel.
The second case enables plasma vapor depositing conditions to be maintained as the substrate band moves through the vessel. In order to achieve the usually desired high degree of uniformity it is necessary, however, to monitor the deposition rates and temperature gradients along the path of the substrate movement. This information is needed in attempts to control such rates and gradients in order to compensate for variations in the deposition rate and morphological structure of the coatings which would vary continuously since the substrate passes the cathode or target area only once.
In prior continuous coating apparatus, besides the problems raised by the fact that the substrate passes the target area only once, other problems arise which produce nonuniformity in the thin film being deposited. One of the causes of such problems is mechanical variation in the distance between the substrate and the target and the other is shifting of the plasma envelope. The variation of distance between substrate and target can be caused by inability to maintain the substrate perfectly flat while it is in the plasma as for instance where it passes over wheels or drums which are not perfectly concentric. Substrate which is delicate and flexible will buckle and bubble. The shifting of the plasma envelope could be due to shifting of the current densities along the path of the substrate as it moves through the vessel. The uneven consumption of the target material is a common reason for this since the current will seek the paths of least resistance in the plasma between the target and the substrate support.
It is obvious that the optical properties of a photographic film for example will be unpredictable unless absolute uniformity can be maintained. In the case of an article which uses a photoelectrically responsive thin film or one which is to be used in microelectric circuitry, variations in the thickness of the deposit can render the article practically useless.
In addition to the above problems, a change in temperature of the entire pressure vessel or of a localized area of the substrate will cause major problems affecting both the local deposition rate and the morphological structure in that area. For example, one may have an amorphous deposit of materials in a one inch diameter region surrounded by sizable crystals which grow in those areas where the substrate was inadequately cooled. Buckling or bulges in the substrate could cause this also.
The throughput rate in a plasma vapor deposition apparatus is dictated by the size of the target, the deposition rate and the required thickness build-up which in turn determines the velocity or exposure time of the substrate. In the prior art targets of enormous length are being utilized in some instances to subject the substrate member to a long exposure time while it travels along the target. This method is primarily used in depositing thin films on long ribbons of substrate materials as in the manufacture of capacitors. Metal foils are deposited on plastic ribbon substrate. The technique is faulty because of extreme difficulty in maintaining uniformity in the characteristics mentioned - yet the object of the long target is to enable sufficient coating in one pass of the substrate member through the vessel. The throughput rate is nevertheless slow. Increasing the throughput rate decreases the uniformity and also because of a deposition rate established by current densities results in thinner coatings. For example, a target which operates at a deposition rate of 10A of a given material per second per square inch will deposit 20A per square inch if exposed two seconds. A speed-up of the travel velocity of the substrate member causes a reduction of the exposure time and hence results in a thinner coating. Half a second will provide only 5A per square inch of total thickness deposited.
The method and structures of the invention were developed for a purpose for which techniques and structure of the prior art were deficient. The film which is contemplated to be manufactured by the invention is described in some detail in the first said copending application, Ser. No. 260,848 entitled "ELECTRO-PHOTOGRAPHIC FILM." The invention herein is not limited to this purpose, however, but has a wide application in the art generally -- being especially useful where great uniformity in thickness and stoichiometry is required in a coated substrate which is to be made in large quantities at great economy.
The electrophotographic film described in the above-identified first mentioned application utilizes a photoelectrically sensitive coating which is intended to be charged in darkness, then exposed to a light image at high speed, then treated with a toner to fix the light image with which it is exposed. The electrical characteristics of the film such as its electronic gain and conductivity are exponentially related to the inherent thickness of the coating and its variations. In other words, normally attainable thickness tolerance will not suffice to meet the speed and conductivity criteria of the electrophotographic film.
The invention herein provides methods and structure for obviating the problems which have been inherent in the prior art methods and apparatus.